Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/469

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Dr. Mac Culloch on Quartz Rock.

sketch to which I refer, will doubtless also remind the spectator of the view given in Dr. Fitton's paper, of the hills in the vicinity of Dublin. The conical form of these hills appears to arise partly from the rapidity of their decomposition, as far as regards their mountainous bulk, and partly from the permanent nature of the resulting fragments. To the same cause also is owing that particular and arid appearance of sterility, which is so characteristic of this class of mountains, an appearance which those who have seen Jura will readily recognize, combined with a difficulty in ascending them, which the geologist who has laboured in the attempt will not easily forget. The peculiar whiteness which the surfaces of the fragments show is however adventitious, as the fresh rock exhibits a variety of grey, yellow, and brown tints, which long exposure to the atmosphere will ultimately bleach. A narrower examination of unaltered specimens would possibly have prevented mineralogists from ever applying to this rock the improper designation of granular quartz, how much soever the weathered surfaces may resemble this substance.

On a nearer inspection, these mountains appear to consist of a stratified rock, or to be formed of various beds of grit, which, where the declivities are so steep as not to admit the lodgment of fragments, become easily visible. In my remarks on Jura, I have described a distinct and continuous bearing of the strata, but I was unable by the aid of a spying-glass to perceive at any of the various points from which I viewed the mountains of this coast, a similar tendency through any considerable space. I should conceive their tendencies to be exceedingly various, and that no general system of inclination or direction predominated among them. But of this I must needs speak with much diffidence, as I had no opportunity of ascending any of the summits, from whence alone an accurate observation of this nature could be made. Those who have been occupied in

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